The most expensive and the most time-consuming part in setting up conventional cable-based telecommunication networks is cabling, which connects each subscriber to the local exchange. In many cases it is preferable to replace fixed subscriber cables with a radio connection which can be established easily and quickly to connect the subscribers to the public switched telephone network (PSTN) in areas where no subscriber lines are available and it is too expensive, too slow or otherwise impractical to install them. Thus the subscriber network comprises, in the same way as a normal mobile telephone network, base stations connected to the exchange, the subscriber terminals (mobile telephones) having a radio connection with the base stations. Such a system is called a wireless local loop (WLL). Since a WLL radio system is generally an extension of the normal public switched telephone network, it typically uses the normal numbering of a phone network. Alternatively, it is possible to use the numbering of a mobile telephone network. A call to the phone number of a WLL subscriber generally causes the routing of the call always to one and the same exchange and base station connected to it, the base station paging the subscriber via a radio path. It is possible to connect any conventional telephone set to the WLL radio system, and thus the radio path is invisible to the user and provides, through the base station, a point-to-point connection between the WLL telephone set and the exchange.
The WLL network operator is usually responsible for the equipment at both ends of the radio path. The base station equipment is connected directly to the network and thus it is relatively easy to monitor. However, the operator should also be able to monitor the subscriber terminal at the other end of the radio path. If the level or quality of the signal received by the subscriber terminal is too low, the quality of the call is poor or calls to the subscriber terminal will be completely prevented. This corresponds to a poor or a disconnected subscriber line in a fixed network. In addition, in WLL applications the operator cannot provide power supply for the subscriber terminal through the subscriber line as in a cabled phone network, but the power supply has to be organized locally in the location of the subscriber terminal. This means that the subscriber terminal has to be provided with a battery backup unit to ensure the connection in all situations, for example during a power failure. Therefore, it should be possible to monitor also the power supply of the subscriber terminal, the charge level of the battery backup unit and preferably also any malfunctions related to the supply of charging current.
Mobile telephones for conventional mobile telephone networks conduct, in an idle state, some kind of measurements on the received signal level and quality of the radio channel. A typical mobile telephone also measures the charge level of the battery. These measurements are conducted, however, only for the user of the mobile telephone, who is responsible for the condition of his own mobile telephone in a conventional mobile phone network. The measured information is shown to the user on the display of the mobile telephone, and if some failure is detected, the user is alarmed. U.S. Pat. No. 5,142,563 discloses a similar application, where the portable part of a wireless telephone monitors the voltage level of its battery power source, and when the voltage level drops, sends a signal to the fixed part, the so-called base station, of the wireless telephone, the signal directing the base station to a quick-charging state to wait for the wireless telephone to be positioned on the charging device of the base station. The base station is meant to be connected to a normal subscriber line of a wired telephone network. The operator of a conventional mobile telephone network or a fixed network is not responsible, however, for the working condition of the wireless terminal equipment or the mobile telephone of the subscriber, but, instead, what is provided is only a local indication about the condition of the terminal equipment, given to the user or intended for the internal use of the terminal equipment. Such a local indication is not suitable, however, for the operator of a WLL network, since its utilization would require repeated visits to the mobile telephone or the wireless telephone, and would thus be extremely difficult and would mean long intervals between the checks. The operator of a WLL network needs such information centralized in one place, so that the operator could control the accessibility of the network and the need for maintenance.